JPS5855237B2 - silver metal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a silver plating method using a silver plating solution with low or no cyanide by treating the substrate to be plated with a mercaptan and preventing immersion plating of silver onto the substrate. or a mercaptan compound soluble in the solution in an amount sufficient to substantially move the cyanide-free silver plating solution.

Electrodeposition (electroplating) of silver and its alloys has been carried out for a long time, and is particularly used in decorative applications.

Silver electroplating is also quite widely adopted in the electrical and electronic industries in addition to and instead of gold electroplating.

One of the problems with electroplating silver is that most metal substrates that are desired to be silver plated are plated with silver during dipping.

This silver immersion plating occurs when no charge has been transferred.

This silver dip-plated layer has poor adhesion to the substrate, resulting in an electroplated product with poor adhesion.

Therefore, when electroplating silver, it is common practice to first deposit a very thin layer of silver on the surface of the substrate in a process known as a so-called silver strike.

This silver strike solution typically has a low silver content and a high free cyanide content.

Using such solutions with low silver content and high free cyanide content, immersion plating on substrates, such as copper substrates, is significantly reduced or does not adhere sufficiently to the substrate. Then, even if silver is electroplated on top of it, a plated product with suitable adhesion can be obtained.

Thus, having a substrate with a thin strike layer of silver can be electroplated using conventional silver electroplating solutions with high silver contents.

Since the silver is not dip plated onto the silver, coating the strike layer with a second or higher layer of silver is done with complete or good adhesion.

The problems of immersion plating silver on the surface of a substrate are even more pronounced when attempting to electroplate silver by spot plating methods or using high speed silver plating solutions.

Although the individual plating machines in use today differ mechanically in many ways, the silver plating baths found in them are very similar in operation, and the plating solution is sprayed at a high velocity. It is ejected from an outlet, nozzle or similar type orifice onto the plated surface.

This results in vigorous agitation in a limited area of the cathode section, which is usually desirable, and results in high-speed plating of pure silver on a limited plating surface.

This method not only requires the use of a silver strike solution prior to the final electrodeposition of silver, but also eliminates the silver build-up that occurs on the metal substrate piece during the off-cycle of the spot plating process. The problem of immersion plating will be encountered.

For example, using an apparatus such as that disclosed in U.S. Pat. No. 3,723,283 (Johnson et al.), a spot silver strike would occur at station S9 and silver electroplating would occur at station 811. It will be done.

At point 811 (see FIG. 4), electrolyte is constantly pumped into the header.

When the header is in the "on" position, coupon 10 is suitably shielded and current flows through contact with the electrodes, electroplating the unshielded portions of the coupon.

When strip 10 is moved to the next location for electroplating, the header is released, unshielding the entire strip and cutting off the current.

However, the electrolyte still continues to splash onto the strip and immersion plating of silver occurs over the unshielded gold strip.

Such immersion plating of silver not only inhibits the desired electrical properties but also clearly results in the loss of valuable metal.

Typically, silver cyanide electrolytes deposit a dull, dull white layer of silver on the substrate, even if the substrate has been previously strike plated, and requires polishing to obtain a shiny plated product. be.

This problem can be solved by using a brightener (bright) in the silver cyanide plating solution.
This has been overcome to some extent by the prior art technique of adding (tening).

Tellurium, selenium and sulfur compounds have been proposed as brighteners for such solutions.

This sulfur compound is one of the compounds mainly used commercially today.

The addition of sulfur compounds to conventional silver cyanide solutions, which contain large amounts of free cyanide, is used as a brightening agent, resulting in a high-gloss coating that requires little or no polishing. A silver-plated product will be produced.

However, these solutions with added sulfur compounds also cause silver plating during immersion onto the metal substrate, so a silver strike solution is required to maintain proper adhesion during silver electroplating. It is.

Examples of commonly used silver solution and sulfur compound brighteners include U.S. Pat.
No. 3,517, No. 2,800,439, No. 3.362
, No. 895, No. 3,446,716 and No. 3.58.
No. 0,821, and "Electroplated Silver and Its Alloys, Electroplating and Metalworking", June 1976, pages 3-5 and 8-13 (EL
BCTRODEPO8ITEDSILVERAND I
TS ALLOYS, ELECTR-OPLATING
AND METAL FINISHING).

Cyanide-type high-speed plating solutions, which contain little, if any, free cyanide, also have the disadvantage of immersion plating the silver onto the substrate, thus reducing the desired adhesion to the final silver plating. A silver strike is said to be preferable to obtain the desired characteristics.

A number of non-cyanide silver plating solutions are known and have been proposed for use in low speed plating as well as high speed plating, but none have reached the commercial stage.

Thiosulfate silver plating solutions are one example, but they also require silver striking in a different type of solution, i.e., a silver cyanide solution with more free cyanide and less silver. Therefore, its uses are limited.

Attempts to use thiosulfate solutions that are high in free thiosulfate and low in silver have shown that this solution also causes immersion plating of silver, which leads to silver immersion in acid-based silver thiosulfate solutions. It is not successful because it inhibits adhesion.

The present invention provides a low cyanide or Concerning a silver plating solution that does not exist at all.

The present invention also provides methods for treating or coating a substrate to be plated with a compound having mercapto groups, and then applying the treated substrate to a silver electrolyte with or without a compound soluble in a solution having mercapto groups. It relates to a method of treating with a plating solution.

As previously mentioned, solution-soluble sulfur compounds, including mercaptan compounds, have previously been used as brighteners in cyanide-rich silver cyanide solutions.

Addition of these soluble sulfur compounds to free cyanide-rich silver cyanide solutions does not prevent dip plating of silver onto metal substrates when the substrates are immersed in the solution.

High-speed and low-speed solutions that do not have high free cyanide content are also disclosed.

Examples of such solutions are described in U.S. Pat. No. 4.024.031 (
Lerner: Disclosed in Lerner.

As also disclosed in the Lerner patent, these solutions also deposit silver onto the metal substrate during immersion, thereby interfering with the adhesion of the silver electroplated layer.

We have proposed that prior to silver electroplating the substrate be treated with a mercaptan or a non-cyanide solution or a solution containing only a limited amount of free cyanide, such as that disclosed in the Lerner patent. The present invention has been completed based on the discovery that addition of a soluble mercaptan compound prevents or substantially impedes immersion plating of silver onto a substrate.

The mercaptan compounds in these solutions have a very impressive effect in interfering with silver immersion plating, allowing silver to be electroplated directly onto the substrate without silver strike plating, and with good adhesion. are doing.

Addition of these mercaptan compounds to normal free cyanide-rich solutions has the effect of imparting brightness as disclosed in the prior art, but does not prevent or prevent silver dip plating.

According to the method of the present invention, the same mercaptan compound, when added to a non-cyanide silver solution or a low cyanide silver solution, produces a compound that prevents or substantially impedes silver immersion but does not provide the same brightness as the prior art solutions. It has no effect.

The present invention also provides a non-cyanide silver solution, such as a silver-thiosulfate or silver-thiocyanate strike solution, containing an amount of a mercaptan compound sufficient to prevent or substantially impede dip plating of silver onto a metal substrate. It also includes a strike solution.

Although silver-thiosulfate complex strike solutions with low silver content are known, these solutions cause immersion plating of silver onto the metal substrate, thereby interfering with the adhesion of the final conventional silver electroplating. do.

To use the silver-thiosulfate complex electroplating solution,
Previously, it was necessary to use a strike solution with a low silver content and a high cyanide content in order to obtain sufficient adhesion with silver plating using conventional high concentration silver thiosulfate solutions.

Addition of the silver-thiosulfate strike solution hemercabutan compound prevents or substantially impedes dip plating of silver onto conductive substrates and facilitates electroplating of silver onto metal substrates by a completely cyanide-free process. It allows plating and eliminates all the obstacles associated with silver cyanide plating or strike solutions.

The silver plating solution of the present invention can contain various electroplatable silver compounds or complexes as disclosed in the prior art.

Preferred silver compounds include alkali silver cyanides, such as KAJ(CN)2.

The amount of silver present in the solution (bath) may be the amount normally used in low speed and high speed solutions.

Generally, it is preferable for high-speed solutions to have a high silver content;
Particularly high A/m2 (A/ft2) (hereinafter referred to as ASM (AS)
F').

) is used.

For example, an ASM of about 8.608 to about 1,076 (800
Approximately 60 g/l of silver metal is preferably used in high velocity solutions using an ASF of -100.

The temperature of the solution can vary widely.

It can be operated at room temperature, but at approximately 38 to 66°C (1o
It is preferred to operate the high speed solution at 0-150'F.

The pH of the high-speed plating solution is normally the pH used by those skilled in the art.
That's fine.

8-9.5 pH is preferred. Although lower pH values can be used, solutions containing high concentrations of silver are unstable if the pH is too low.

Higher pH values can be used as well, but if the pH is around 9.5
When this is reached, free cyanide accumulates in the solution, creating control problems.

Substrates to be silver plated by the method of the invention are copper and copper alloys.

The prucaptan compound soluble in the plating bath used in the present invention may be any compound having a norcapto group that can be dissolved in the plating bath. derivatives, norcapto derivatives of organic acids such as thiomalic acid, thiolactic acid, etc.

The concentration of these mercaptan compounds in the solution necessary to prevent or substantially prevent dip plating of silver onto the substrate depends in part on the particular solution used and the particular mercaptan compound used; Determined by routine experimentation.

In the case of a basic solution, for example, as shown in the example, thioglycerol is about 0.25 ml/11. Thiomalic acid is about 0.6 g/l and thiolactic acid is about 0.15 Tl1
Effective at l/13.

As long as the mercaptan is dissolved in the solution, it is natural that it can be used at a higher concentration, but since it is sufficiently effective even at a lower concentration, there is no need or advantage to use it at a higher concentration.

According to previous experiments, thiolactic acid is preferred.

According to the present invention, the free cyanide content that may be present with the mercaptans that prevent or impede immersion plating of silver is determined to some extent in the basic silver solution used together with the particular mercaptan compound and its amount used. Dependent.

The best results to date have been with silver plating solutions containing very little or no free cyanide, and releasing free cyanide from the solution as it is produced in-system by silver metal detachment. obtained when operating the solution at a pH such as

The amount of free cyanide that can be tolerated can be determined by routine experimentation on individual systems, and previous experiments have shown that using a basic solution as described in Example 1 and 0.15 ml of thiolactic acid. It has been found that maintaining free cyanide below approximately 10 g/l results in very little silver dip plating which would interfere with adhesion.

The main use of these plated products is in the semiconductor field.
Involves die attach and die attach.

For this purpose, it is not necessary to make the plated object shiny.

Although the silver plated article and plating solution produced by the present invention can be used as is, the addition of a selenium compound to the plating solution improves the smoothness of the plated article and allows it to be plated at higher current densities.

As previously mentioned, selenium compounds are commonly known as brighteners in high cyanide silver plating solutions.

An example of this is US Pat. No. 2,110,792.

However, the effect as a brightening agent is not so great, and other known brightening agents used in ordinary high cyanide silver solutions do not exhibit the expected brightening effect like mercaptan compounds. It is not something that can be inferred to.

Other components that can be added to the solution according to the invention include surfactants, conductive agents, buffers, such as borates, citrates, -basic alkali phosphates, dibasic alkali phosphates, etc. There are obvious benefits for businesses.

The fact that the silver strike can be omitted, the adhesion of the plating material is good, and silver is hardly plated onto undesired substrates by immersion makes it possible to use the solution according to the present invention in the field of high-speed plating for the production of semiconductors. making it highly desirable for commercial operations.

The present invention also includes the addition of a non-cyanide silver strike solution hemelcabutan compound to prevent dip plating of silver.

An example of such a solution is a silver thiosulfate alkali complex strike solution.

These strike solutions usually contain less than about 5 g/l of silver metal, free conductive agents such as alkali sulfates and alkali citrates.

The use of mercaptan compounds in conventional silver thiosulfate solutions with high contents of silver metal and free alkali sulfate does not prevent immersion plating of silver.

For example, about 30 g/l of silver metal as alkali silver thiosulfate complex and about 250 g/l of free sodium thiosulfate and about 20 g/l of sodium sulfite N
Approximately 0.15 rul/l in a solution containing a2803
Addition of thiolactic acid does not prevent silver immersion plating.

Viewing the invention in this light, silver thiosulfate strike solutions are also necessary for commercial operations.

If a substrate such as copper is first strike plated with a mercaptan-containing silver thiosulfate strike solution, dip plating will not occur and the silver strike layer will adhere well to the substrate, allowing subsequent standard thiosulfate strikes as described above. The layer can be thickened by conventional silver electroplating with a silver sulfate solution.

If the present invention is viewed from this perspective, the strike solution cannot be omitted, but it has an important purpose in that it can omit the need to use a silver cyanide solution to obtain a silver strike plating layer that adheres sufficiently to the substrate. accomplish

Immersion plating of silver can also be performed using non-cyanide solutions such as silver cyanide solutions and silver thiosulfate strike solutions containing less than about 10 g/l of free cyanide by first treating the substrate to be plated with a mercaptan compound. It can be prevented in a silver oxide solution.

The substrate can be dipped into the mercaptan solution for a few seconds and then strike plated with a silver thiosulfate strike solution, or directly silver plated with a free cyanide-poor silver cyanide solution.

The mercaptan solution can contain anything <0.05 percent or more.

Do not coat or contaminate the immersed substrate.

Therefore, the amount of mercaptan and soaking time must be related to avoid coating or contaminating the substrate.

The mercaptan solution can also contain large amounts of silver, such as 60 g/l and 90 g/13 dipotassium nonate.

The substrate can then be plated with a mercaptan-free silver solution.

Silver strike solutions, whether cyanide or non-cyanide, usually contain about 5 j! 7
Conventional plating solutions or high silver content silver plating solutions usually contain at least about 15 g/l of silver metal, whereas conventional plating solutions or high silver content silver plating solutions usually contain at least about 15 g/l of silver metal.

Accordingly, a silver strike solution or a silver-lean solution according to the present invention contains less than about 5 g/l of silver metal, and a silver plating solution contains at least about 15 g/l of silver metal.

Example 1 A basic silver plating aqueous solution was prepared containing 90 g/l dipotassium phosphate and 60 g/l silver metal as soluble alkali silver cyanide.

Add 0.15 ml/l of thioemulsion to this basic solution and use conventional high speed, spot or jet plating at a temperature of about 600 C (about 140'F) and an ASM of about 8.608 (ASF of about 800). Silver plating was performed on a copper substrate using the method.

Although the resulting silver plating was not shiny, its properties, such as adhesion to substrates, were excellent for the desired electrical applications.

No silver precipitation was observed during immersion.

When silver plating was performed using only a basic solution, a considerable amount of silver was deposited by immersion.

Example 2 II) Same procedure as Example 1 except that 11 m KCNSe was added together with thiolactic acid.

The same results as in Example 1 were obtained, and a smoother plating layer was obtained.

Examples 3 and 4 Two separate basic solutions as described in Example 1 were prepared.

To one solution 0.25 rrtl/l thioglycol was added and to the other solution 0.6 g/l thiomalic acid.

The two solutions were each used to silver plate in the same manner as described in Example 1, with almost identical results.

Examples 5 and 6 The solutions of each of Examples 3 and 4 were silver plated as described in Example 1 with the addition of an additional 1 pl) III of KCNSe in addition to the hemercabutan compound.

Almost the same results as those obtained in Examples 3 and 4 were obtained, but the plating was smoother and somewhat glossier.

Reference example 7 30 g/l sodium thiosulfate in water, 30.9/l
A silver thiosulfate strike aqueous solution was prepared by mixing 19/l of sodium citrate and 19/l of silver metal as a sodium thiosulfate complex salt.

When this strike solution was used, silver was deposited on the copper substrate due to immersion, resulting in poor adhesion.

Thiolactic acid at 0.15 m/A' was added to this solution.

No precipitation of silver occurred during immersion, and adhesion was good.

Reference Example 8 An aqueous solution of 0.05 weight percent thiolactic acid was prepared,
The cleaned copper substrate was dipped into it for a few seconds.

The thus treated substrate was then strike plated with the silver thiosulfate solution of Example 7 before addition of thiolactic acid.

No silver dip plating was observed, and the adhesion of silver strike plating was good.

A similarly treated copper substrate was silver plated with the basic silver cyanide solution of Example 1.

No silver immersion plating was observed, and the adhesion of the silver plating to the base material was good.

Claims (1)

[Scope of Claims] 1. The electrolytic plating bath (a) contains not more than 10 g/l of free cyanide, and (b) the mercaptan compound prevents or substantially eliminates immersion plating of silver onto the substrate. Conductive copper or copper alloy from a silver cyanide alkaline electrolytic plating bath containing at least one soluble mercaptan compound selected from the group consisting of mercapto derivatives of polyglycols and mercapto derivatives of organic acids in an amount sufficient to inhibit conductive copper or copper alloys. A method for electrodepositing silver on a conductive copper or copper alloy substrate, which comprises electrolytically plating the substrate. 2. The electrodeposition method according to claim 1, wherein the mercapto derivative of polyglycols or the mercapto derivative of organic acids is thioglycerol, thiomalic acid or thiolactic acid. 3. The electrolytic plating bath contains at least 15 g/13 of silver as alkali silver cyanide, and the electrolytic plating bath is sprayed continuously at high speed onto the surface of the substrate to be plated. The electrodeposition method according to item 1 or 2.